1. Field of the Invention
The present invention relates to a method for writing reference data in a sub storage region of a semiconductor storage device and to a semiconductor storage device which has the reference data written therein.
2. Description of the Related Art
Memory devices used for various types of electronic devices for handling digital information can be largely classified into a group of storage devices such as hard disk drives, DVDs, and CDs, which require physical operations; and another group of storage devices equipped with a semiconductor memory device which requires no physical operations. The semiconductor memory device can be further classified into two types according to the method of sustaining stored data. More specifically, the semiconductor memory device can be classified into a volatile semiconductor memory device in which stored information will vanish when the power supply is turned off and a nonvolatile memory device which can sustain the stored information even when the power supply is turned off.
The nonvolatile memory device has one charge accumulation portion for one memory cell and sustains stored information in a manner such that the status in which no charge is accumulated in the charge accumulation portion or an amount of charge less than a predetermined amount is accumulated therein (the not-stored status) is defined as “1”, whereas the status in which an amount of charge equal to or greater than the predetermined amount is accumulated in the charge accumulation portion (the stored status) is defined as “0”. Some of those memory cells have, for example, an n-type MOSFET (Metal Oxide Semiconductor Field Effect Transistor) structure, in which a FG (Floating Gate) of polycrystalline silicon that is electrically isolated from the other portions is buried in a gate oxide film of the MOSFET. Furthermore, some other memory cells have an n-type MOSFET structure, in which the gate insulating film of the MOSFET is constructed such that a nitride film is sandwiched between oxide films (i.e., an oxide film, a nitride film, and an oxide film are sequentially deposited). In particular, such a structure is referred to as the MONOS (Metal Oxide Nitride Oxide Silicon) structure or SONOS (Silicon Oxide Nitride Oxide Silicon) structure. The floating gate and the nitride film correspond to the charge accumulation portion. Now, by taking the memory cell of the MONOS structure as an example, a description will be made to data write, readout, and erase operations on such a memory cell. Note that in the data write, readout, and erase operations below, the drain (drain terminal and drain region) and the source (source terminal and source region) are defined such that the drain and the source that are determined when stored data is read are also defined as the drain and source even for the write and erase operations. That is, in the data write, readout, and erase operations, the drain and the source that are determined when stored data is read remain the same all the time, and thus the drain and the source are not to be interchangeably defined for the data write and erase operations.
To write data “0” in the nitride film, a positive voltage is applied to the source terminal and the gate terminal with the drain terminal at the ground voltage. This allows electrons travelling from the drain region to the source region through the channel to gain high kinetic energy in the vicinity of the source region and be turned to hot electrons, so that the positive voltage applied to the gate causes the hot electrons to be raised to immediately below the gate terminal, and the raised electrons are sustained in the nitride film. The amount of hot electrons that is equal to or greater than a predetermined amount is sustained in the nitride film, thereby allowing data “0” to be written.
To read the data stored in the nitride film, a positive voltage is applied to the drain terminal and the gate terminal with the source terminal at the ground voltage. At this time, when no charge is accumulated in the nitride film or an amount of charge less than a predetermined amount is accumulated therein (i.e., data “1” is stored in the nitride film), a comparatively large readout current is obtained. On the other hand, when an amount of charge equal to or greater than a predetermined amount is accumulated in the nitride film (i.e., data “0” is stored in the nitride film), the readout current is reduced due to the charge accumulated therein when compared with the status in which data “1” is stored. As such, since the magnitude of the readout current differs depending on the amount of charge in the nitride film, it is possible to read data by determining the magnitude of the readout current. As a detailed method for determining the magnitude of readout current, data is written in advance in a predetermined memory cell of a nonvolatile memory device so that an intermediate amount of charge (i.e., the predetermined amount that is mentioned above) is accumulated therein, the intermediate amount of charge taking on an intermediate value between the amount of charge accumulated in the nitride film of the memory cell in which data “1” is stored and the amount of charge accumulated in the nitride film of the memory cell in which data “0” is stored. Then, the current flowing through the memory cell in which data has been written in advance (hereinafter, also referred to as the sub storage region or the reference cell) is compared with the current flowing through another memory cell (i.e., a memory cell to be read (hereinafter, also referred to as the main storage region)). If the current flowing through the another memory cell is greater than the current flowing through the reference cell, it is determined that data “0” is stored in the another memory cell. If the current flowing through the another memory cell is less than the current flowing through the reference cell, it is determined that data “1” is stored in the another memory cell.
To erase the data stored in the nitride film, a positive voltage is applied to the source terminal and the ground voltage or a negative voltage is applied to the gate terminal with the drain terminal opened. This allows hot holes occurring in the vicinity of the source region to be injected into the nitride film to neutralize the charge accumulated in the nitride film, thereby erasing the data.
Disclosed in Japanese Patent Kokai No. 2005-64295 (Patent Literature 1) is a memory cell in which the charge accumulation portion formed of a nitride film being sandwiched between oxide films is disposed on the drain terminal side and the source terminal side of the gate electrode.